Wireless communication systems are often vulnerable to fading and other effects, which may cause the amplitude of received signals to vary. Consequently many wireless receivers employ automatic gain control (AGC) circuits which attempt maintain an output scaled signal at a substantially constant amplitude or power level, despite variations in a received signal.
Conventional AGC circuits typically include a variable-gain amplifier, an envelope or power detector connected to the output of the variable-gain amplifier, and a comparator connected to the output of the detector. The gain of the variable-gain amplifier varies with a gain control input signal supplied to the amplifier. Consequently, the amplifier produces an output signal whose amplitude varies with the gain control input signal. The detector measures the amplitude or power of the signal output from the amplifier. The comparator compares the detector output with a reference signal, and normally supplies a difference signal to the variable-gain amplifier as the gain control input signal.
Recently, AGC circuits are being implemented using digital electronics. A typical digital AGC circuit includes an analog variable-gain amplifier, an analog-to-digital converter (ADC) connected to the output of the amplifier for converting the analog input signal into digital form, a digital amplitude or power detector connected to the output of the ADC for estimating the amplitude or power of the signal output from the ADC, a digital gain controller connected to the output of the detector for calculating the appropriate gain for the variable-gain amplifier, and a digital-to-analog (DAC) connected to the output of the DSP for supplying either a gain value or a gain control value as an analog gain control input signal to the variable-gain amplifier.
Whether implemented using analog components or some combination of digital and analog components, AGC arrangements provide for the use of other receiver components, such as ADCs, having more limited dynamic range than an overall desired operating dynamic range of a receiver. For example, if a receiver with digital signal processing functions operates in an environment where input signal level has a large dynamic range, then either AGC or high resolution ADCs must be used. Unfortunately, the use of high resolution ADCs increase the manufacturing cost and power consumption of the receiver. Although AGC would provide for the use of lower resolution ADCs, the aim of AGC is to maintain a scaled signal within a desired dynamic range, such that the amplitude of the scaled signal output from a variable-gain amplifier or gain stage does not tend to vary significantly, as discussed above. This substantially constant-power signal output from the gain stage is suitable for communication system receivers that use hard decision processing. However, soft decision processing arrangements, which can improve the performance of some receivers, cannot be easily implemented with AGC. Soft decision processing requires absolute signal level information for received signals.
Therefore, there remains a need for a communication device that provides for soft decision receiver processing in a receiver having AGC.
There remains a related need for a system and method for inverting AGC to thereby provide absolute signal level information for soft decision processing of a received communication signal.
There remains a further need for such a system and method that performs soft limiting of absolute amplitude information.